With their high energy density and high capacity, nonaqueous electrolyte secondary batteries, typified by the lithium ion secondary battery, are widely used as the power source for portable electronic equipment such as portable telephones, portable personal computers and portable music players, and further as the drive power source for hybrid electric vehicles (HEVs) and electric vehicles (EVs).
For the positive electrode active material of these nonaqueous electrolyte secondary batteries, use is made, either singly or mixed together, of LiCoO2, LiNiO2, LiNixCO1-xO2 (x=0.01 to 0.99), LiMnO2, LiMn2O4, LiNixMnyCozO2 (x+y+z=1), LiFePO4, or the like, all of which are able to reversibly absorb and desorb lithium ions.
Of these, particularly frequent use is made of the lithium-cobalt composite oxides or the dissimilar metallic element-supplemented lithium-cobalt composite oxides, since these give superior battery characteristics relative to the others. However, cobalt is expensive and exists in small amounts as a resource. Therefore, in order to continue using these lithium-cobalt composite oxides or dissimilar metallic element-supplemented lithium-cobalt composite oxides as the positive electrode active material in nonaqueous electrolyte secondary batteries, it is desired to raise the performance of nonaqueous electrolyte secondary batteries to even higher levels.
However, when a nonaqueous electrolyte secondary battery in a charged state is stored under high temperature conditions, the positive electrode is prone to degrade. This is considered to be because when a nonaqueous electrolyte secondary battery is stored in a charged state, there will occur oxidative decomposition of the nonaqueous electrolytic solution on the positive electrode active material, or elution of the transition metal ions of the positive electrode active material, and furthermore, under high temperature conditions, such decomposition of the nonaqueous electrolytic solution and elution of transition metal ions will be faster than under room temperature.
Meanwhile, JP-A-2004-179146 sets forth a case where, with the purpose of enhancing the cycling characteristics, battery capacity, high temperature storage characteristics, etc., of a nonaqueous electrolyte secondary battery, a small amount of dinitrile compound is added to the nonaqueous electrolytic solution. US Patent Publication No. 2008/0220336 sets forth a case where, with the purpose of improving the high temperature storage characteristics of a nonaqueous electrolyte secondary battery, a nonaqueous electrolytic solution that contains cyclic carbonate and dinitrile compound is used. Furthermore, JP-A-2008-108586 sets forth a case where, with the purpose of obtaining a high-capacity nonaqueous electrolyte secondary battery with superior charge-discharge cycling characteristics and storage characteristics, dinitrile compound is added to the nonaqueous electrolytic solution.
JP-A-2009-32653 sets forth a case where, with the purpose of curbing production of gases during high temperature storage and improving the cycling characteristics in a nonaqueous electrolyte secondary battery, a nonaqueous electrolytic solution is used that contains at least one compound selected from the group consisting of a compound containing from two to four nitrile groups in its structural formula; a fluorinated cyclic carbonate having two or more fluorine atoms; monofluorophosphate; and difluorophosphate.
PCT Publication No. WO2006/038532 discloses a separator that has the advantages that its impregnability with a nonaqueous electrolytic solution, mechanical strength, permeability, and high temperature storage characteristics when used in a battery, are enhanced; namely a separator constituted of a polyolefin microporous membrane constituted of two or more stacked films containing polyethylene and polypropylene, in which in one or both of the surface layers, the content of polypropylene-containing organic particles is not less than 5% by mass and not more than 90% by mass.
With the inventions disclosed in JP-A-2004-179146, US Patent Publication No. 2008/0220336, and JP-A-2008-108586, dinitrile compounds are essentially adsorbed onto the positive electrode active material in the charged state, and this is deemed to enable protection of the surface of the positive electrode active material and reduction of side reactions between the nonaqueous electrolytic solution and the positive electrode active material, and to have the effect of enhancing the various battery characteristics during high temperature storage. However, according to the results of experiments by the present inventors, when dinitrile compound is added to the nonaqueous electrolytic solution in a quantity sufficient to exert an enhancing effect on the high temperature storage characteristics, the internal resistance of the positive electrode active material is observed to increase and the cycling characteristics to decline. It is inferred that this is because the protective film is formed excessively on the surface of the positive electrode active material.
In JP-A-2009-32653, a cycling characteristic enhancement effect up to roughly 100 cycles, due to dinitrile compound, fluorinated cyclic carbonate, and fluorophosphate being contained in the nonaqueous electrolytic solution, is acknowledged, but it cannot be said that an adequate cycling characteristic enhancement effect is exerted. Hence, it is difficult to achieve both adequate high temperature storage characteristics and cycling characteristics simply by adding dinitrile compound to the nonaqueous electrolytic solution. Although in PCT Publication No. WO2006/038532, a separator is disclosed that is able to improve the high temperature storage characteristics of a nonaqueous electrolyte secondary battery, nothing is disclosed concerning cycling characteristic enhancement effects.
The present inventors conducted many and various investigations concerning the conditions under which it is possible to achieve adequate high temperature storage characteristics and cycling characteristics by adding a small amount of dinitrile compound to a nonaqueous electrolytic solution that has long been in common use. As a result, they arrived at the present invention upon discovering that, when used in combination with a separator of a particular structure among the separators disclosed in JP-A-2009-32653, it is possible to achieve adequate high temperature storage characteristics and cycling characteristics by adding a small amount of dinitrile compound to a nonaqueous electrolytic solution that has long been in common use.